In semiconductor technologies, masks (photomasks or reticles) are formed with predesigned integrated circuit (IC) patterns. The masks are used to transfer those predesigned IC patterns to semiconductor wafers in lithography processes. Any defect on the masks will be transferred to semiconductor wafers and cause yield issues and quality concerns. In addition, particulate contamination is a source of mask defects.
One type of particulate contamination is mask haze, which may be introduced during mask making, handling, or lithography processes. For example, chemicals, such as SO2 and/or NH3, may be outgassed during mask making process. Despite some cleaning procedures, these chemicals continue to exist at the surface of the masks during lithography exposure process, such as deep ultraviolet (DUV) exposure process. The DUV exposure process generates highly energized photons that facilitate reaction of these chemicals with atmospheric gases to produce ammonium sulfate nano-crystals, causing mask haze.
Mask haze may cause printing errors on a wafer. Therefore, it is desirable to prevent mask haze to be formed onto the masks.